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Organic Chemistry Lab > Distillation > Flashcards

Distillation Flashcards

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1
Q

Why must the thermometer bulb be below the sidearm on the simple distillation apparatus? Can the bulb touch the side of the glass of apparatus? Why?

A
  • So it accurately records the temperature of vapor distilling through the sidearm.
  • No, the bulb cannot touch the side of the glass of apparatus because it would take the temperature of the glass, not the vapor
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2
Q

Why must the end of the sidearm be inside the receiver vial?

A
  • To minimize vapors from escaping out results in a loss of liquid.
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3
Q

How do you control the heating of the apparatus?

A
  • Do this by using a spatula to scrape the sand both around or away from the flask.
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4
Q

Why do you use a boiling chip? Why can you only use the boiling chip once?

A
  • Boiling chips prevent superheating.

- Boiling chips cannot be reused because the pores would fill with liquid.

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5
Q

How do boiling chips prevent superheating?

A
  • Boiling chips provide nucleation sites which allow the liquid to boil smoothly.
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6
Q

Why do you use a wet paper towel to cover the condenser (sidearm)?

A
  • To ensure vapor condenses within the sidearm.
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7
Q

Why do you wrap the apparatus is tin foil?

A
  • To insulate and minimize heat loss.
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8
Q

What are the two characteristics of simple distillation?

A
  1. ) It is not a complete separation.

2. ) Both liquids are purer than before but are not completely pure.

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9
Q

What is the best criteria for deciding to use simple distillation?

A
  1. ) When the boiling point difference between the two liquids is large ( more than 75 degrees C)
  2. ) When the composition of one liquid is less than 10%
  3. ) When the liquid has a non-volatile component (like a solid contaminant).
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10
Q

What are the best criteria for fractional distillation?

A
  1. ) Boiling point differences between the two liquids is small.
  2. ) The composition of the two liquids is substantial.
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11
Q

What is fractional distillation?

A
  • It is a series of continuous distillations.

- After a fraction vaporizes, some compound condenses and gets redistilled.

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12
Q

Why do you need to supply more heat to the fractional distillation apparatus vs. the simple distillation apparatus?

A
  • Because the vapor has to travel a farther distance before reaching the condenser in fractional distillation.
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13
Q

What are the differences in apparatus’ between fractional and simple distillation?

A
  • Fractional distillation apparatus is longer and has a copper sponge inside its distilling column.
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14
Q

What are the 5 reasons why a copper sponge is packed in the distilling column of the fractional distillation apparatus?

A
  1. ) Easy material to pack within
  2. ) Does not come out of the column-like beads do
  3. ) Great heat transfer characteristics
  4. ) Low hold up
  5. ) Has a large surface area.
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15
Q

What is hold up?

A
  • Unrecoverable distillate that wets the column packing.
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16
Q

Why must the column be vertical in fractional distillation?

A
  • So the condensing liquid can percolate through the rising hot vapors.
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17
Q

What happens if the upper portion of the column is at a consistently higher temperature than the lower portion of the column?

A
  • The liquid condensate will vaporize as it moves down the column.
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18
Q

What are the 4 important requirements to perform successful fractional distillation and describe them?

A
  1. ) Significant contact between the liquid and vapor phases in the column.
    - Achieve this by packing the column with something that has a large surface area.
  2. ) Maintenance of the proper temperature gradient along the column
    - This determines the rate of distillation so don’t distill too fast.
  3. ) Sufficient length of the column
    - Figured out by trial and error
  4. ) Sufficient difference in boiling points between the two liquids
    - Should be about 20-30 degree difference.
    - If a smaller difference then the length of the column and/or type and amount of packing material must be adjusted.
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19
Q

What happens if you distill too slow?

A
  • A column will get flooded which is characterized by a column of liquid observed in the fractional column at the joint between column and reaction flask.
  • Make sure the rate of distillate is adjusted.
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20
Q

Why must the rate of distillation be slow?

A
  1. ) So the ascending vapors and descending liquids are in equilibrium.
  2. ) To maintain a proper temperature gradient in the column.
  3. ) Overheating can lead to bumping of liquid in the flask.
  4. ) Underheating can lead to reflux (when vapors condense and return to distilling flask instead of distilling).
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21
Q

What does the column need to be to ensure proper heating? And what is it?

A
  • Needs to be adiabatic (heat doesn’t leave or add to the system)
  • Insulate apparatus with foil.
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22
Q

What are the 3 precautions of distilling? Describe them.

A
  1. ) Never distill in air tight system (closed system)
    - Apparatus may rupture.
  2. ) Never distill to dryness.
    - Dry liquid residue can be explosive.
  3. ) Do not fill flask more than 2/3.
    - This may cause the liquid to pump and contaminate distillate.
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23
Q

What are 3 sources of error?

A
  1. ) Loose connectors
    - Will cause vapor to escape.
  2. ) Temperature variations
    - Due to airflow in hoods
  3. ) Large surface area of microscale apparatus.
    - May loose heat to the environment.
24
Q

What are the 4 reasons why you may not recover all the liquid to be purified/separated?

A
  1. ) The positioning of the sidearm.
  2. ) Loose connectors and adapters
  3. ) Hold up in fractional distillation
  4. ) A significant amount of distillate left adhering to the glass surface of the apparatus.
25
Q

How do you calibrate the thermometer specifically for carrying out distillation or checking boiling points?

A
  • You check the 0 degrees C point using crushed ice and distilled water.
  • You also check the 100 degrees C point using boiling water.
26
Q

What is the use of distillation?

A
  • To purify liquids using boiling points.
27
Q

What must the 2 liquids be in order for fractional and simple distillation to work?

A
  • They must be a homogenous layer, miscible in one another.
28
Q

When do liquids boil?

A
  • When the vapor pressure of liquids=atmospheric pressure as the liquids are heated.
  • Higher altitude = lower pressure = lower boiling point
29
Q

Why doesn’t all liquid evaporate when reaching the boiling point?

A
  • Need to account for the heat of vaporization.
30
Q

Why are boiling points not as good of indicators of purity as melting points are?

A
  • This is due to vapor pressures which measure how easily molecules escape the surface of liquids.
  • Liquids with higher vapor pressures have lower boiling points when compared with liquids with lower vapor pressures.
31
Q

What are the two characteristics when distilling a pure liquid?

A
  1. ) The boiling point of liquid=vapor temperature.

2. ) The temperature remains constant throughout distillation.

32
Q

What are the characteristics of distilling mixtures?

A
  1. ) The temperature does not remain constant; it increases throughout distillation because:
    (a. ) the composition of vapors that is distilling varies throughout.
    (b. ) some lower boiling point component comes off first then the vapor is enriched with higher boiling point component.
33
Q

In mixtures what would happen if the sum of the partial pressures = the prevailing atmospheric pressure?

A
  • The liquid would boil
34
Q

What happens if the packing material in fractional distillation is too dense?

A
  • The column would have pressure changes which would cause nonequilibrium conditions.
35
Q

What happens if the packing material has too large a surface area?

A
  • It can hold too much of the material being distilled and also cause pressure changes causing nonequilibrium conditions.
36
Q

What is HETP? And what is its relationship to the efficiency of the column?

A
  • Height equivalent to a theoretical plate is each successive condensation-vaporization cycle, also called a theoretical plate.
  • The smaller the HETP, the more plates the column will have and the more efficient it will be.
37
Q

What are azeotropes?

A
  • Mixtures with the fixed composition that cannot be altered by simple or fractional distillation.
38
Q

What are the characteristics of azeotropes?

A
  1. ) Will act as a pure compound even though it is a mixture.
  2. ) B.P will remain constant throughout distillation.
  3. ) The composition of the liquid = composition of vapor.
  4. ) On the boiling point composition graph, the liquid and vapor curves pinch together at the azeotropes composition.
39
Q

What are the two types of azeotropes?

A
  1. ) Can be minimum boiling (below the pure boiling point of both liquids.
  2. ) Can be maximum boiling (above the pure boiling point of both liquids.
40
Q

What are two methods to form 100% ethanol from 95% ethanol? What type of azeotrope is this?

A
  1. ) The water can be removed chemically (by a reaction with calcium oxide).
  2. ) Water can be removed as an azeotrope by adding benzene and distilling it.
    - This is a minimum boiling azeotrope.
41
Q

What is vacuum distilation? When is it used and for what compunds is it used?

A
  • Distillation carried out under reduced pressure.
  • Used when ordinary or steam distillation are not practical.
  • Used for compounds that decompose at boiling points or sensitive oxidation.
  • Liquids must be miscible.
42
Q

In vacuum distillation why must glassware be checked for scratches or cracks?

A
  • This may cause implosion under the vacuum.
43
Q

When is steam distillation used?

A
  • Usually with immiscible liquids (one of them being water)
44
Q

What is a characteristic of steam distillation?

A
  • The boiling point of the mixture is below the boiling point of each pure compound.
45
Q

What are the advantages of steam distillation?

A
  1. ) Volatizing liquids and solids below their boiling points by passing steam into a boiling flask.
  2. ) This is good for high boiling substances that decompose before boiling point is reached.
46
Q

What requirement is necessary so that the boiling point remains constant steam distillation?

A
  • Adequate amounts of both water and organic component are present to saturate vapor space
47
Q

When boiling sugar and water what is the temperature in the vapor phase vs. the temperature in the sugar solution itself?

A
  • The vapor phase is at 100 degrees C, while the sugar solution itself is above 100 degrees C and continues to rise as the concentration of sugar in the remaining solution increases.
48
Q

What is the vapor pressure of the sugar solution dependent on?

A
  • The number of water molecules present in a given volume.
  • With increasing nonvolatile sugar molecules and decreasing concentration of water, the vapor pressure at the given temperature decreases and a higher temperature for boiling increases.
  • Sugar molecules do not leave the solution and the drop clinging to the thermometer is pure water in equilibrium with pure water vapor.
49
Q

Does boiling point change with pressure?

A

Yes.

50
Q

What actions do you take to specifically heat a test tube?

A
  1. ) A boiling stick is easier to remove than a boiling chip but use a boiling chip instead of a boiling stick in a reaction.
  2. ) You can hold the test tube in your hand because it is long and narrow.
  3. ) The sand is hotter deeper down and cool on top providing a range of temperatures.
51
Q

Why do you need to dilute the solution in filtration in a Pasteur pipette? And how does this process work?

A
  • So the solute will not recrystallize at about 22 degrees C.
    1. ) Place cotton in the tip of the pipette and use another pipette to put the solution within this pipette.
    2. ) Use the pipette to filter out the solution and use a fresh solvent to rinse the pipette and cotton.
    3. ) The filtered solution is then concentrated by evaporation.
52
Q

What does a change in boiling point during distillation mean for a pure liquid?

A
  • Means there is an impurity present.
53
Q

Does a constant boiling point indicate only one compind is present?

A
  • No.
    1. ) You could have two miscible liquids that have similar boiling points. OR
    2. ) Azeotropes mixtures have constant temperatures that are below or above the boiling points of the individual components.
54
Q

How does structure affect boiling point? (3)

A
  1. ) The strength of the intermolecular forces
    - Stronger intermolecular forces the lower the boiling point.
    - Ionic > Hydrogen > Dipole-Dipole > VDW
  2. ) Boiling point increases with molecular weight.
  3. ) Branching decreases boiling point
55
Q

How does structure affect boiling point? (3)

A
  1. ) The strength of the intermolecular forces
    - Stronger intermolecular forces the lower the boiling point.
    - Ionic > Hydrogen > Dipole-Dipole > VDW
  2. ) Boiling point increases with molecular weight.
  3. ) Branching decreases boiling point
56
Q

How does atmospheric pressure affect boiling point?

A
  • The boiling point is reached when the vapor pressure of liquid = atmospheric pressure.
  • If atmospheric pressure increases, boiling point increase.
  • If atmospheric pressure decrease, boiling point decreases.

Organic Chemistry Lab (5 decks)

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